1. Field of the Invention
The present invention relates to a conveying device that has a conveying means to transport a print medium on a conveying belt by holding it to the belt by suction, and to a printing apparatus using the conveying device.
2. Description of the Related Art
A conveying device that has been proposed in conventional printing apparatus to keep a surface of a print medium from coming into contact with a print head uses a suction device that attracts the print medium to an endless conveying belt by suction.
FIG. 7 is a perspective view of a conveying device disclosed in Japanese Patent Laid-Open No. 2007-302406. An endless conveying belt 41 has a plurality of suction holes piercing therethrough in a direction of the belt thickness and is wound around a drive roller 43 and a follower roller 44, arranged parallel to each other at the same height. On an inner circumferential side of the conveying belt 41 there is provided a platen 42 with a suction chamber formed therein. A negative pressure generating means, such as a fan (not shown), generates a negative pressure in the suction chamber inside the platen 42 to produce a suction force on the surface of the conveying belt 41 through the suction holes in the conveying belt 41. The conveying belt 41 attracts the print medium by the suction force generated as described above.
To perform an accurate printing on a print medium fed onto the conveying belt 41, the printing apparatus needs to detect the front end of the print medium.
FIG. 8 shows a continuous label sheet as an example of print medium. In detecting the front end of a continuous sheet such as shown in the figure and a cut sheet, a method is available which uses a transmission type sensor to detect the front end of the print medium. The transmission type sensor comprises a light emitting portion and a light receiving portion and performs the front end detection by the light from the light emitting portion passing through the label sheet at its base portion 45 but being cut off at a label portion 46.
FIG. 9 shows another example of the continuous sheet, a tag sheet 47 with no area through which the light from the light emitting portion is allowed to pass. As shown in FIG. 9, the tag sheet 47 has marked portions 48 on its print surface, and on the back side the marked portions 48 are coated in advance with a color that absorbs light. The front end detection is made by detecting the state of light of the reflection type sensor that is reflected from the marked portions 48 and from other areas.
In an inkjet printing apparatus required to produce a high quality print, the distance between the print surface of the print medium and the ejection port surface of the print head is desired to be kept as small as possible. This distance needs to be kept at a predetermined short distance in order to maintain a stable printing operation.
In a printing apparatus that prints on a continuous sheet such as a label sheet, a so-called back-feed operation to return the front end of the print medium, after being cut, to the position of the front end detection sensor is required. If the front end detection sensor is installed remote from the print head, the back-feed distance becomes long, raising a possibility of jamming. Further, locating the front end detection sensor remote from the print head also can increase the print medium conveying distance and therefore the size of the conveying device. So, the front end detection sensor needs to be installed near the print head.
Where the transmission type sensor installed near the print head is used to detect the front end of the print medium after the print medium is transported on, and attracted by suction to, the endless conveying belt formed with suction holes, the following problem arises. One of the light emitting portion and the light receiving portion of the transmission type sensor needs to be installed on one surface side of the print medium and the other on the opposite side. So, the light emitting portion must throw light on other than the area of the print medium that is in contact with the conveying belt.
However, the portions of the print medium which are not in contact with the conveying belt may curl or deform due to moisture and the printing operation of the print medium. The curling or deformation can cause the portions of the print medium that are not in contact with the conveying belt 41 to float, with the result that the floating portions may come into contact with the ink ejection port face of the print head. This in turn gives rise to a problem of degraded print quality and possible damages to the print head.
Where the reflection type sensor is used to detect the front end of the print medium, marked portions are provided on the back side of the print medium opposite the print surface to allow the front end detection to be made based on the state of the light reflected from the marked portion and from other areas. So, the reflection type sensor needs to be provided on the back side of the print medium, i.e., on the side where the conveying belt is laid. That is, the sensor detection area of the print medium must not be in contact with the conveying belt, which means that the aforementioned problem can also arise with the reflection type sensor.
Further, although the endless conveying belt can be formed as one integral part, as shown in FIG. 7, a plurality of narrow conveying belts may be arranged parallelly side by side in a direction crossing the conveyance direction for the purpose of reducing the cost. In such a case, the aforementioned problem may also occur in other than the areas where the print medium is not in contact with the conveying belt.